Over at the Rutgers blog there’s “an interesting discussion”:http://dtaatb.weebly.com/1/post/2010/04/bummed-about-botzmann-brains-buy-in-to-babyverses.html about the various puzzles that arise given the conjunction of orthodox views on statistical mechanics combined with some indifference principles.

This seems to me to be another reason to not like indifference principles, to go alongside the various reasons I gave “a few years ago”:http://brian.weatherson.org/evil.pdf. To be sure, I think to get out of this argument you need a slightly stronger hostility to indifference principles than I give in that paper. In the paper I argued that the following argument failed at step 3.

1. The situation between us and other entities that are phenomenally like us is epistemically symmetric.
2. Epistemically symmetric situations should be treated in a cognitively symmetric ways.
3. If you should treat some situations as cognitively symmetric, then you should give them equal credences.
4. So you should give equal credence to situations that are phenomenally alike.

If you like non-numerical credences, then you should think step 3 is clearly false. But I think steps 1 and 2 alone create some sceptical sounding results in the cases that quantum mechanics brings up.

So we should focus a little on step 1. Is it really true that phenomenal equivalence implies epistemic equivalence? That seems false for three reasons.

One reason concerns speckled hen type cases. Two people who are phenomenally alike might get different amounts of justification if one is better at tracking/observing fine details of their apparent environment, such as the number of speckles on an observable hen.

Another reason concerns history. Even if you’re a phenomenalist about evidence, it’s plausible that our evidence consists of a stream of phenomena, not just our apparent phenomena. In “Elusive Knowledge”, for instance, David Lewis says he takes our phenomenal history to be given, and not something threatened by sceptical doubts.

The biggest reason is that we may well be externalist about evidence. As Timothy Williamson has argued, when it comes to (apparent) perception, it’s plausible to identify evidence with what we know. Williamson extends this to all evidence, though I don’t think this is a particularly plausible. But that doesn’t matter – if we have different evidence from someone in a sceptical scenario, then some arguments for scepticism don’t get going.

The upshot of all this, I think, is that a serious study of the nature of evidence in epistemology seems to be important for, among other things, physics. Sean Carroll and David Albert have been defending theories whose defence relies on principles most epistemologists would reject. If the epistemologists are right, physics may be a little simpler than some physicists think.